Tile expert witnesses at Ceramic Tile and Stone Consultants write in the Ceramic Tile & Stone Industry News:
The substrate is the foundation of a ceramic tile or stone installation. When this foundation is unsuitable for whatever reason, then the products applied on top are automatically in jeopardy. Thankfully, remedies exist for correcting substrate problems, but it is important to first evaluate the substrate and take any corrective action as part of the floor preparation process.
Compounded by the shortage of qualified installers, most ceramic tile and stone (Tile & Stone) floor failures are related to the lack of proper floor preparation. These floors are only as good as the substrate to which they are applied and the method of installation used. Substrate preparation, in turn, will determine whether a particular substrate is adequate for the intended use.
Common failures are due to substrate cracking and excessive deflection, moisture problems (that can lead to mold), bond failure, and lack of flatness or slope.